Heinlein is overrated. He's also inconsistent in quality, one out of ten books being truly good. Many others would be passible if his writing didn't resemble someone getting distracted by a shiny object and suddenly realizing that 300 pages of ramble have been written. Plus, I'm really sick of hearing about Lazarus Long's hairy thighs.
I love Orson Scott Card, Larry Niven, James Morrow, Greg Bear (though there was this one book he wrote that was so bad I never forgave him), Rita Mae Brown, and Arthur C Clarke. I consider Card, Clarke, and Bear a holy trinity with Morrow as the antichrist.
There's other writers I really like, but those are the ones that really wowed me.
And I have read more than scifi, duh. But people like Blake, Frostic, Milton, Eliot, and Twain are all debatable in quality, depending on who likes what.